The present invention deals with imparting motion to a test object such as a motor vehicle in a controlled fashion; apparatus and methods are involved.
Known vehicle shakers usually have hydraulic wheel loader actuators which sometimes have an additional piston/cylinder in parallel with the hydraulic piston/cylinder to provide a “load bias” capability. This additional parallel cylinder uses gas such as nitrogen with a simple control system to provide static loads, thereby reducing the burden from the hydraulics. Such actuators are commonly used in 4-post and 7-post vehicle shaker rigs. The industry standard for 7-post shaker rigs is to use three hydraulic body loaders with integral “compliance link” elements in addition to four hydraulic wheel loaders. Body loaders are also referred to as aeroloaders or downforce actuators. The compliance link is essentially a soft spring in series with the hydraulics. Some body loader designs use pneumatic actuation instead of hydraulics, based on a belief that the pneumatic cylinder would naturally act as a soft spring to provide performance equivalent to the industry standard, but less expensive to operate. However, such pneumatic body loader technology does not have enough control bandwidth to meet the needs of a typical 7-post or 8-post test. The industry standard hydraulic body loader solution has a higher bandwidth, but it also suffers from bandwidth limitations that are created by the presence of the “compliance link.”